The invention relates to processes for free radical polymerization using transition metal superoxides as initiators.
The superoxide radical anion O2xe2x88x92. is an active oxygen species that possesses both anionic and free radical properties. It is of particular interest in biological systems, where the superoxide dismutase enzyme catalyzes the dismutation of O2xe2x88x92. to H2O and O2. because it appears to be involved in a variety of oxidation reactions. A variety of other applications for superoxide compounds are known. Commercially, potassium superoxide is utilized in self-contained breathing equipment for generation of oxygen gas. U.S. Pat. No. 4,731,197, for example, describes one such system. U.S. Pat. No. 4,101,644 discloses the use of calcium superoxide for the same function. Superoxide compounds have also been used to oxidize organic compounds for a variety of purposes. Purification of acetal monomers using alkali metal superoxides is described in U.S. Pat. No. 4,513,144. Detoxification of polyhalogenated organic compounds using superoxide is disclosed in U.S. Pat. No. 5,358,657. Dewkar et al. (Angew. Chem. Int. Ed., 40, pp 405-407 (2001)) employed titanium superoxide for the conversion of aromatic primary amines directly to nitro compounds. In polymeric systems, Osawa et al. (J. Polym. Sci., Polym. Chem. Ed., 19, pp 1877-1884 (1981)) depolymerized vinyl acetate polymers with potassium superoxide. Han et al. (J. Polym. Sci., Part A, 29, pp. 281-286 (1991)) describe use of potassium superoxide as an initiator for anionic polymerization of monomers having electron withdrawing substitutents: nitroethylene, acrylonitrile and acrolein. Monomers lacking such substituents, including methyl methacrylate and styrene, were unreactive toward the anionic superoxide initiator.
Anionic polymerization processes variously termed xe2x80x98living,xe2x80x99 xe2x80x98controlledxe2x80x99 or xe2x80x98immortalxe2x80x99 may be used to synthesize polymers having a narrow molecular weight distribution and low polydispersity (xe2x89xa61.5). These processes are so named because polymerization generally occurs by addition of monomer units to a constant number of growing polymer chains until all monomer has been consumed; if more monomer is added, polymerization resumes. Molecular weight is controlled by the stoichiometry of the reaction, and is typically a linear function of conversion. Block copolymers with well-defined morphology prepared by such processes are of significant commercial importance. These living anionic processes are necessarily limited to use with monomers that can polymerize by an anionic mechanism, but many commercially important monomers do not undergo anionic polymerization under convenient conditions. Therefore, more recently, living free radical polymerizations have been investigated for monomers that polymerize by a free radical mechanism. In one example, International Application No. WO 99/01478 discloses use of dithioester chain transfer agents along with standard free radical initiators, including azobisisobutyronitrile and benzoyl peroxide, for living free radical polymerizations. However, there remains a need for new initiators of free radical polymerization, and, particularly, for living free radical polymerization.
In addition, contamination of polymers by initiator residues can negatively affect many desirable polymer properties, including, for example, thermal stability, color retention (or lack thereof), and water and/or solvent resistance or sensitivity. Heterogeneous initiators have not been used for free radical polymerization in the art, and, consequently, there is a need for initiators that can be readily separated from the polymer produced.
A new class of free radical initiators, transition metal superoxides, has been unexpectedly discovered that can initiate free radical polymerization, under conditions commonly used in free radical polymerization processes. These transition metal superoxides are solids, and are insoluble in most solvents, both aqueous and organic. In addition, polymers having a narrow molecular weight distribution and low polydispersity can be synthesized using these initiators. For the purposes of the present application, the term xe2x80x98transition metalxe2x80x99 refers to, in the periodic table, elements 21 through 29 (scandium through copper), 39 through 47 (yttrrium through silver), 57 through 79 (lanthanum through gold), all known elements from 89 (actinum) on, in addition to aluminum, gallium, indium and tin. In particular, titanium, tungsten, vanadium, and zirconium superoxides may be used.
In one aspect, the present invention relates to a free radical polymerization process comprising combining at least one monomer polymerizable by free radicals and at least one transition metal superoxide of formula M(O2)n, where M is a transition metal and n is equal to the valence of M; generating free radicals from said transition metal superoxide; and polymerizing said at least one monomer. In particular, the metal may be Ti, W, V, or Zr. At least one chain transfer agent or molecular weight controlling agent may be with the monomer and transition metal superoxide. In particular, the chain transfer agent may be a dithiocarboxylic ester of formula I: 
wherein R1 is a m-valent radical selected from the group consisting of alkyl, substituted alkyl, haloalkyl, thioalkyl, heteroalkyl, substituted heteroalkyl, aryl, substituted aryl, haloaryl, thioaryl, substituted thioaryl, heteroaryl, substituted heteroaryl, alkylaryl, haloalkylaryl, thioalkylaryl and substituted thioalkylaryl;
R2 is selected from the group consisting of alkyl, substituted alkyl, haloalkyl, thioalkyl, heteroalkyl, substituted heteroalkyl, aryl, substituted aryl, haloaryl, thioaryl, substituted thioaryl, heteroaryl, substituted heteroaryl, alkylaryl, haloalkylaryl, thioalkylaryl and substituted thioalkylaryl; and
m is an integer from 1-6.
More particularly, the chain transfer agent may be a compound of structure II, III, or IV, or a combination thereof: 
wherein
R3 is hydrogen or haloalkyl;
R4 is hydrogen or alkyl;
R5 is hydrogen, haloalkyl or carboxy;
R6 and R11 are independently hydrogen, alkyl, alkoxy, cyano, halo, or carboxy; and
R7, R8, R9 and R10 are independently hydrogen, alkyl, cyano, aryl, or arylcarboxy.
In the context of the present invention, alkyl is intended to include linear, branched, or cyclic hydrocarbon structures and combinations thereof. Lower alkyl refers to alkyl groups of from 1 to 4 carbon atoms. Examples of lower alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, s- and t-butyl. Preferred alkyl groups are those of C20 or below. Cycloalkyl is a subset of alkyl and includes cyclic hydrocarbon groups of from 3 to 8 carbon atoms. Examples of cycloalkyl groups include c-propyl, c-butyl, c-pentyl, and norbornyl. Alkoxy or alkoxyl refers to groups of from 1 to 8 carbon atoms of a straight, branched, cyclic configuration and combinations thereof attached to the parent structure through an oxygen. Examples include methoxy, ethoxy, propoxy, isopropoxy, cyclopropyloxy, and cyclohexyloxy. Lower alkoxy refers to groups containing one to four carbons. Aryl means a 5- or 6-membered aromatic ring; a bicyclic 9- or 10-membered aromatic ring system; or a tricyclic 13- or 14-membered aromatic ring system; each of which rings is optionally substituted at 1-3 positions with lower alkyl, substituted alkyl, substituted alkynyl, xe2x95x90O, xe2x80x94NO2, halogen, hydroxy, alkoxy, OCH(COOH)2, cyano, xe2x80x94NR1R2, acylamino, phenyl, benzyl, phenoxy, benzyloxy, heteroaryl, or heteroaryloxy; each of said phenyl, benzyl, phenoxy, benzyloxy, heteroaryl, and heteroaryloxy is optionally substituted with 1-3 substituents selected from lower alkyl, alkenyl, alkynyl, halogen, hydroxy, alkoxy, cyano, phenyl, benzyl, benzyloxy, carboxamido, heteroaryl, heteroaryloxy, xe2x80x94NO2 or xe2x80x94NRR (wherein R is independently H, lower alkyl or cycloalkyl, and xe2x80x94RR may be fused to form a cyclic ring with nitrogen). The aromatic 6- to 14-membered carbocyclic rings include, for example, benzene, naphthalene, indane, tetralin, and fluorene. Arylalkyl means an alkyl residue attached to an aryl ring. Examples are benzyl and phenethyl. Substituted alkyl, aryl, cycloalkyl, or heterocyclyl refer to alkyl, aryl, cycloalkyl, or heterocyclyl wherein up to three H atoms in each residue are replaced with alkyl, aryl, haloalkyl, halogen, hydroxy, lower alkoxy, carboxy, carboalkoxy, carboxamido, cyano, carbonyl, nitro, amino (primary, secondary or tertiary), alkylthio, sulfoxide, sulfone, acylamino, amidino, phenyl, benzyl, heteroaryl, phenoxy, benzyloxy, or heteroaryloxy.
In another aspect, the present invention relates to a process for the preparation of a transition metal superoxide comprising combining 30% hydrogen peroxide and a transition metal precursor comprising a transition metal compound or complex that is soluble in an aqueous solvent system. The transition metal precursor may be a soluble transition metal alkoxide, oxyalkoxide, aryloxide, oxyaryloxide, or a complex thereof. The transition metal may be titanium, tungsten, vanadium, or zirconium.
In yet another aspect, the present invention relates to a transition metal superoxide produced by the process of described above.
In still another aspect, the present invention relates to composition comprising a transition metal superoxide of formula M(O2)n, wherein M is a transition metal other than titanium; and n is equal to the valence of M. In particular, M may be tungsten, vanadium or zirconium.